The features of the present invention may be used in the printing arts and more particularly in electrophotographic printing. In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is image wise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image in a region known as the development zone. Toner particles are attracted from carrier granules or bead carriers of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a copy sheet and permanently affixed thereto. The foregoing generally describes a typical mono-color electrophotographic copying machine.
Recently, electrophotographic printing machines have been developed which produce highlight color copies. A typical highlight color printing machine records successive electrostatic latent images on the photoconductive surface. When combined, these electrostatic latent images form a total latent image corresponding to the entire original document being reproduced.
One latent image is usually developed with black toner particles. The other latent image is developed with color highlighting toner particles, e.g. red toner particles. These developed toner images are transferred sequentially to the copy sheet to form the color highlighted copy. A color highlight printing machine of this type is a two pass machine.
Single pass highlight color printing machines using tri-level printing have also been developed. Tri-level electrophotographic printing is described in detail in U.S. Pat. No. 4,078,929. As described in this patent, the latent image is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level electrophotographic printing, the charge on the photoconductive surface is divided in three, rather than two, ways as is the case in mono-color printing. The photoconductive surface is charged, typically to about 900 volts. It is exposed image wise, such that one image corresponding to charged image areas remains at the full potential of 900 volts. The other image, which corresponds to discharged image areas is exposed to discharge the photoconductive surface to its residual potential of typically about 100 volts. The background areas are exposed to reduce the photoconductive surface potential to about halfway between the charged and discharged potentials, (typically about 500 volts). The developer unit arranged to develop the charged image areas, is typically biased to about 600 volts, and the developer unit, arranged to develop the discharged image areas, is biased to about 400 volts.
The single pass nature of this system dictates that the electrostatic latent image pass through the developer units in a serial fashion. When the latent image, which has a high charged image potential region and a low charge image potential region, passes through the first developer unit, arranged to develop the discharged image areas, an extremely high cleaning field potential is established between the electrically biased developer unit and the highly charged image areas of the latent image. This high cleaning field potential attracts developer material from the developer unit onto the highly charged image areas. When this occurs, the highly charged image areas of the electrostatic latent image are locally discharged where developed, and as a result, white spots will be noticeable in the solid area images developed by the second developer unit, which at the present time is black, rendering the prints unacceptable.
This problem was overcome by the invention disclosed in the below-referenced co-pending U.S. application Ser. No. 07/604,269. However, electrostatic forces and adhesion forces within the developer units contribute to a condition where the bead carriers are carried out of the development unit. Bead removal devices (BRDs) are well known and commonly used to pick off any developer carriers which are carried out of the development zone of a development unit.
Generally BRDs operate by generating a strong magnetic field in the area between the photoconductive surface and the BRD to attract free bead carriers to the shell of the BRD. These captured beads are then deposited in a sump or developer receiver as the shell of the BRD is rotated. This arrangement, however, renders release of beads from the BRD more difficult, e.g. gravity and centripetal forces often are insufficient to achieve release of the beads from the magnetic field as the shell of the BRD rotates. That is the strong magnetic field necessary to attract bead carriers from the photoconductive surface to the BRD shell are sufficiently strong around the shell itself to retain some bead carriers as the shell rotates.
As disclosed in U.S. Pat. No. 4,829,338, the magnetic field from the magnet positioned in the shell of the BRD can be directed by use of a ferromagnetic shunt to promote bead removal from the photoconductive surface while enhancing the field between the photoconductive surface and the BRD to attract free beads. Nevertheless, this solution has not overcome all problems associated with BRDs.
Specifically, some bead carriers remain attached to the shell as the shell rotates. These unreleased beads tend to attract additional beads to themselves to form bead chains. These bead chains can span the gap between the BRD and photoconductive surface causing degradation of the image being reproduced. For example, in highlight color electrographic printing machines, when bead chains occur in the first discharged developer unit, portions of the second latent image which are contacted by the chains are discharged thereby causing discharge line defects in the finished print.
Also, the arrangement disclosed in U.S. Pat. No. 4,829,338 does not address carrier pickup and carry through in reverse rotation of a BRD. In highlight color electrographic printing machines using magnetic delivery means, monochromatic images are often achieved by disabling one of the developer units. One technique is to reverse the direction of the magnetic brush rollers as disclosed in U.S. Pat. No. 4,811,046 to May, which is incorporated by reference herein. The reversal of the direction of travel of the rollers in the developing units in this case effects a substantial reduction in developer available to effect the charged photoconductive surface. Such reversal of angular travel of the rollers can also be done during warm-up and shut down cycles to remove stray or extraneous developer materials from the developer zone. However, reversed rotation of the rollers is not entirely satisfactory, as some beads are not released and certain other beads are attracted to and attach to the roller. Failure to release the beads, as well as the attraction of additional beads, results, for example, in discharge of portions of the latent image on the photoconductive surface as it passes through a developer unit to a secondary developer unit.
During ordinary operation some beads fail to release from the roller. When this involves a substantial number of beads, a condition mimicking a low toner condition results. Essentially, empty beads are being transported through the development zone and insufficient toner is delivered to develop the latent image.
Optimally the magnetic field at the release point of development rollers must be minimized to reduce carry through of `empty` beads and to prevent carry back of beads in the "reversed" rotating rollers. Further, this must be accomplished without substantially affecting the magnetic field characteristics along the remainder of the rollers which are responsible for bead pickup, bead transport, bead timing, developing by toner, migration from bead to photoconductive surface and carry out of beads from development zone. Also, the magnetic fields around a BRD housing must be maximized along the interface between the BRD shell and the photoconductive surface and minimized at all other points to afford return of released beads to a sump or reservoir and to impede bead chain formation.
Various techniques have heretofore been used to develop electrostatic latent images as illustrated by the following disclosures, which may be relevant to certain aspects of the present invention:
U.S. Pat. No. 4,320,958 PA1 Patentee: Fantuzzo PA1 Issued: Mar. 23, 1982 PA1 U.S. Pat. No. 4,641,946 PA1 Patentee: Forbes II PA1 Issued: Feb. 10, 1987 PA1 U.S. Pat. No. 4,833,504 PA1 Patentee: Parker et al. PA1 Issued: May 23, 1989 PA1 Co-pending U.S. application Ser. No. 07/604,269 PA1 Applicant: Hogestyn PA1 Filed: Oct. 29, 1990
The relevant portions of the foregoing patents may be briefly summarized as follows:
U.S. Pat. No. 4,320,958 discloses a processing station for an electrophotographic printing which cleans the photoconductive surface and develops an electrostatic latent image recorded thereon. The processing unit uses an indexable magnet positioned interiorly of a rotating tubular sleeve. During development, the magnet is indexed so that a weak magnetic field is generated in the development zone during development. During cleaning, the magnet is indexed to generate a strong magnetic field in the cleaning zone.
U.S. Pat. No. 4,641,946 describes a developer roller having a rotating tubular sleeve with a magnet disposed interiorly thereof. A photoconductive belt is wrapped about a portion of the exterior surface of the sleeve. The magnet generates a radial magnetic field in the development zone which, at the center, may range from 0 gauss to 500 gauss. FIG. 4 shows a radial magnetic field, in the development zone, having a valley of about -185 gauss and twin peaks, each of about -385 gauss.
U.S. Pat. No. 4,833,504 discloses a single pass highlight color electrophotographic printing machine using two developer units. The first developer unit contains developer with black toner. The black toner is driven to the most highly charged areas of the latent image by the electrostatic field between the photoreceptor and developer rolls. The second developer unit contains developer with the highlight color toner. The highlight color toner is urged towards the parts of the latent image at the residual potential, i.e. the discharged region of the latent image, by the electrostatic field between the photoreceptor and the development rolls in the second housing. The magnetic rolls in the second developer unit are constructed such that the radial component of the magnetic force field produces a magnetically free development zone intermediate a charge retentive surface and the magnetic rolls. The developer is moved through the zone magnetically unconstrained and subjects the image developed by the first developer unit to minimal disturbance. In addition, the developer is transported from one magnetic roll to the next.
Co-pending U.S. patent application Ser. No. 07/604,269 describes an electrophotographic printing machine in which an electrostatic latent image is recorded on a photoconductive surface. One portion of the latent image is a discharged area with the other portion of the latent image being a charged area. The discharged image area is developed with toner particles of a first color and polarity by a first developer unit. The first developer unit generates a weak magnetic field in the development zone and a strong magnetic field at the entrance and exit of the development zone. A second developer unit develops the charged image area with toner of a second color and polarity. The colors of the toners are different from one another.
In accordance with one aspect of the present invention, there is provided a developer unit for use in an electrographic printing device of the type having a latent image recorded on a moving charge retentive surface. The unit includes a developer roller for developing the latent image with toner of predetermined polarity. The developer roller includes a rotatable outer housing, fixed magnetic means disposed within said housing, and ferromagnetic field shaping means disposed within the housing to shape the magnetic fields induced by the magnetic means disposed within said housing. Specifically, the magnetic means and the field shaping means are arranged so bead carriers are attracted and adhere to the outer housing of developer shell for delivery of toner in a development zone and then are released from the shell. Also, the means are arranged so that upon reversal of the roller, the bead carriers do not adhere to the roller for passage through the development zone.
Pursuant to another aspect of the present invention, there is provided a developer unit in an electrographic printing device of the type having an electrostatic latent image recorded on a moving charge retentive surface. The developer unit includes means for developing the latent image with toner of predetermined polarity. The developer unit also includes a bead removal device having a rotatable outer shell and fixed magnetic means positioned within the shell. A first magnetic shunt is disposed in the shell to shape the magnetic field to urge capture of any beads passing between the photoconductive surface and the bead removal device and to urge release of such captured carriers into a sump as the shell rotates. A second magnetic shunt is positioned in the shell to shape the magnetic field to minimize the magnetic fields on a portion of the shell to inhibit bead chain formation. Further, the second magnetic shunt is also positioned to shape the magnetic field to urge release of beads captured during reverse rotation of the BRD shell.
According to yet another aspect of the present invention, there is provided a developer unit in an electrographic printing device of the type having an electrostatic latent image recorded on a moving charge retentive surface. The developer unit includes means for developing the latent image with toner of predetermined polarity. The developer unit also includes a bead removal device having a rotatable outer shell and fixed magnetic means positioned within the shell. A first magnetic shunt is disposed in the shell to shape the magnetic field to urge capture of any beads passing between the photoconductive surface and the bead removal device and to urge release of such captured carriers into a sump as the shell rotates. A second magnetic shunt is positioned in the shell to shape the magnetic field to minimize the magnetic fields on a portion of the shell. The bead removal device may also be connected to an AC power source, and the shell of the magnetic means can be two magnets of substantially opposite polarity orientation.